Seasonal variation in vertical flux of biogenic matter in the marginal ice zone and the central Barents Sea

Abstract

The spatial and seasonal variations in the vertical flux of particulate biogenic matter were investigated in the Barents Sea in winter and spring 1998 and summer 1999. Arrays of simple cylindrical sediment traps were moored for 24 h between 30 and 200 m along a transect from the ice-free Atlantic water to Arctic water with up to 80% ice cover. Large gradients in the quantity and composition of the sinking particles were observed in the south–north direction, and in relation to water column structure and stability, which depend on the processes of ice retreat. The magnitude of the vertical flux of particulate organic carbon (POC) out of the upper mixed layer ranged from background winter values (30–70 mg C m −2 day −1) to 150–300 mg C m −2 day −1 in summer and 500–1500 mg C m −2 day −1 in spring. Vertical flux of chlorophyll a (CHL) was negligible in winter, generally <1 mg m −2 day −1 in summer, and up to 38 mg m −2 day −1 in spring. In spring, the proportion of phytoplankton carbon (dominated by Phaeocystis pouchetii in the Atlantic water and Thalassiosira antarctica in the Arctic water) in the sinking POC was up to 50%. Both colonial and single-celled forms of P. pouchetii were equally abundant in the water column and sediment traps. In contrast to the spring season, the vertical flux of phytoplankton during summer was dominated by a variety of flagellates (e.g. small unidentified flagellates, Ochromonas crenata, Dinobryon balticum and single-celled P. pouchetii). The magnitude of the vertical flux to the bottom in spring was comparable in the Arctic and Atlantic waters (ca. 200 mg C m −2 day −1), but the composition and C/N ratio of the particles were different. The regulation of biogenic particle sedimentation took place in the upper layers and over very short vertical distances, and varied with season and water mass. The vertical flux was mainly shaped by the water column stratification (strong salinity stratification in the Arctic water; no stratification in the Atlantic water) and also by the activity of plankton organisms. Zooplankton faecal pellets were an important constituent of the vertical flux (up to 250 mg C m −2 day −1), but their significance varied widely between stations. The daily sedimentation loss rates of POC in spring exceeded the loss rates in summer on the average of 1.7 times. The complexity of the planktonic community during summer suggested the prevalence of a retention food chain with a higher capacity of resource recycling compared to spring.